The present invention relates to a scanning circuit for reading pixel signals of an image sensor comprised of a number of pixel light sensor circuits each representing a unit pixel and being capable of producing in a photoelectric converting element a photo current proportional to a quantity of incident light falling thereon.
Japan Laying Open Patent Publication No. 2000-32961 discloses a conventional image sensor, in which a pixel light sensor circuit used as a unit pixel comprises, as shown in FIG. 1, a photo-diode PD operating as a photoelectric converting element for producing a sensor current proportional to the quantity of incident light Ls falling thereon, a transistor Q1 having a logarithmic output characteristic in a weak inverse state for converting the sensor current produced in the photodiode into a voltage signal Vpd, a transistor Q2 for amplifying the voltage signal Vpd and a transistor Q3 for outputting a sensor signal So in accordance with a timing pulse of a readout signal Vs and which circuit is characterized by its a wide dynamic range obtained by giving the output a logarithmic characteristic, thereby achieving the high sensitivity of detecting a light signal.
FIG. 2 illustrates an exemplary construction of an image sensor comprising a number of the above-described pixel light sensor circuits arranged to form a matrix of pixels, wherein sensor signals from respective pixels are read by scanning in a time series.
The image sensor shown in FIG. 2 is composed of 4×4 pixels D11-D44 arranged in a matrix of pixel circuits, in which pixel lines are selected one by one with respective selecting signals LS1˜LS4 successively output from a pixel line selecting circuit 1 and pixels in each selected pixel line are readout one by one as respective sensor signals in such a manner that selecting signals DS1˜DS4 successively output from a pixel selecting circuit 2 turn on corresponding switches SW11˜SW14 in a group 3 to read respective pixel signals Vo in a time series. In FIG. 2, numeral 4 designates a power source for gate voltage VG of the transistor Q1 and numeral 6 designates a power source for a drain voltage VD of the transistor Q1. The image sensor is provided with a bias circuit 7 for applying a bias voltage +Vcc through respective reference resistances R1-R4 onto the output sides of respective pixels in one line, whereby respective pixel sensor signals So are output as voltage signals Vo.
The image sensor is also provided with a voltage switching-over circuit 5 to initialize each pixel light sensor signal circuit before detecting a pixel signal Vo by removing electric charge accumulated in a parasitic capacity C of each pixel light sensor circuit by changing a drain voltage VD of the transistor Q1 from a high level H to a lower level L for a specified period under the control of a control circuit (not shown), thereby preventing the occurrence of afterglow of each pixel due to the effect of remaining charge in the parasitic capacity.
A pixel line selecting circuit 1 and a pixel selecting circuit 2 are composed each of a shift-register circuit which is driven synchronously with each other under the control of the control circuit (not shown).
FIG. 3 shows a time chart of the operation of respective portions of the above-described image sensor.
The above-mentioned image sensor however involves such a problem that, when switches SW11-SW14 were turned on to read respective sensor signals Vo from the respective pixels, it takes time for a voltage signal to rise to a value specified by the bias circuit 7 due to a small current capacity of the output transistor Q3 for each pixel, thereby causing variations in pixel sensor signals read in a time series from respective pixels. As the result of this, the image sensor cannot at high speed read respective pixel signals.
To solve the above-described problem, it was attempted to increase the speed of reading the sensor signals by providing a buffer circuit 8 between a group 3 of output switches and a bias circuit 7, as shown in FIG. 4, so that pixel light sensor signals Vo read from the respective pixels can be stored in buffer amplifiers BF1-BF4 during a period of charging each parasitic capacity C in accordance with a photoelectric current for each pixel.
A drawback of this attempt is that it is accompanied by a significant increase in power consumption of the image sensor because of turning on all buffer amplifiers BF1-BF4.
Japanese Laying Open Patent Publication No. H09-93492 also discloses a conventional image sensor in which two shift registers are alternately switched on for reading pixel signals from a charge coupled device (CCD) to create an one-pixel delay in reading two signals. This enables the image sensor to read out pixel signals at an increased speed and output the pixel sensor signals at high stability.
The problem to be solved by the present invention is such that the conventional image sensor using a plurality of pixel light sensor circuits each representing a unit pixel and being capable of producing a sensor signal corresponding to a photo current flowing in a photo-electric converting element requires time for a sensor signal to reach a specified voltage value when reading the sensor signal, causing variations in sensor signals readout in a time series and thereby making it impossible to increase the speed of reading pixel signals.